1. Field of the Invention
This invention relates to a fixing method for the heat-and-pressure fixing of toner images in electrophotographic techniques.
2. Related Background Art
A number of methods as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publication Nos. 42-23910, 43-24748 and so forth are known as methods for electrophotography. In general, copies or prints are obtained by forming an electrostatic latent image on a photosensitive member by various means utilizing a photoconductive material, subsequently developing the latent image by the use of a toner to form a toner image, and transferring the toner image to a transfer medium (recording medium) such as paper if necessary, followed by fixing by the action of heat, pressure, heat-and-pressure, or solvent vapor. The toner that has not transferred to and has remained on the photosensitive member is cleaned by various means, and then the above process is repeated.
In recent years, such copying apparatus have not only been used as copying machines for office working to make copies of originals, but also have begun being used as printers for taking out data values from computers or being used for copying highly minute images such as graphic designs.
As printers, LED printers or LBP printers are prevailing in the recent market. As a trend of techniques, there is a tendency toward higher resolution. More specifically, those which hitherto have a resolution of 240 dpi or 300 dpi are being replaced by those having a resolution of 600 dpi, 800 dpi or 1,200 dpi. Accordingly, with such a trend, the developing systems are now required to achieve a high minuteness. Copying machines have also made progress to have high functions, and hence they trend toward digital systems. In the digital systems, chiefly employed is a method in which electrostatic latent images for toner visible images are formed by using a laser. Hence, the copying machines also trend toward a higher resolution and hence, like the printers, it has been sought to provide a developing system with high resolution and high minuteness. Japanese Patent Application Laid-open Nos. 1-112253 and 2-284158 disclose toners having smaller particle diameters. Accordingly, copying apparatus are severely sought to be more highly reliable. Concurrently therewith, a higher performance has become required, so that any superior machines have become no longer establishable unless the improvement in performances of image-forming apparatus, inclusive of those of toners, can be achieved.
As a means for fixing a visible toner image to a recording medium, a heat-roller fixing system is widely used in which a recording medium holding thereon a visible toner image having not been fixed is heated while it is held and carried between a heat roller maintained at a given temperature and a pressure roller having an elastic layer and coming into pressure contact with the heat roller. A belt fixing system is also known, as disclosed in U.S. Pat. No. 3,578,797.
The heat-roll fixing, however, has the following disadvantages:
(1) A time during which an image-forming operation is prohibited, i.e., what is called a waiting time, is required until the heat roller reaches a preset temperature.
(2) The heat roller must be maintained at an optimum temperature in order to prevent faulty fixing from being caused by the variations of the heat-roller temperature that may occur when the recording medium is passed or because of other external factors, and also to prevent the toner from transferring to the heat roller, i.e., what is called the offset phenomenon. This makes it necessary to make large the heat capacity of the heat roller or heater, and requires a high electric power.
(3) When the recording medium is passed over the heat roller and delivered out, the recording medium and the toner on the recording medium are only slowly cooled because of a high temperature of the heat roller, resulting in a state in which a high adherence of the toner is maintained. Thus, because of the curvature of the roller, there may often occur offset, or paper jam caused by the winding of the recording medium around the roller.
(4) A protective member must be provided so that operators can not directly touch the high-temperature heat roller.
The above problems (1) and (2) in the heat-roll fixing are not fundamentally solved also in the belt fixing system disclosed in U.S. Pat. No. 3,578,797.
Japanese Patent Application Laid-open No. 63-313182 discloses a fixing unit with a short waiting time and a low power consumption, in which a toner visible image is heated via a movable heat-resistant sheet by means of a heating element having a low heat capacity, which generates heat pulsewise by electrification, and is thus fixed to a recording medium.
In a heating-via-film system where an image-fixing sheet is made to pass the surface of a heat roller or film whose surface is formed of a material having releasability to toner while the former""s toner image surface is brought into contact with the latter. Since in this method the surface of the heat roller or film comes into contact with the toner image of the image-fixing sheet, a very good thermal efficiency can be achieved when the toner image is fixed onto the image-fixing sheet, so that the fixing can be carried out rapidly. Thus, this method is effective for electrophotographic copying machines.
However, since in the above method the film surface and the toner image come into contact in a molten state of the latter, part of the toner image may adhere and transfer to the film surface and may re-transfer to the next image-fixing sheet to cause the offset phenomenon to contaminate the image-fixing sheet. It is important for the heat fixing system to be so designed that the toner does not adhere to the film surface.
In recent years, from the viewpoint of environmental problems and protection of resources, reclaimed paper is also being increasingly used year by year. Some reclaimed paper, however, contains additives such as calcium carbonate in a large quantity in order to improve the whiteness of paper. Use of such paper causes a problem that any additive and toner having separated from the paper may accumulate on a pressure member of the fixing unit to appear as stained images (hereinafter xe2x80x9cpressure roller contaminationxe2x80x9d). It has been long awaited to prevent the pressure roller contamination conjointly with making fixing units small-size for the achievement of small-size printers, power saving and also making printers high-speed.
An object of the present invention is to provide a fixing method that can achieve a good fixing performance, promises superior anti-offset properties and also does not cause any pressure roller contamination.
To achieve the above object, the present invention provides a fixing method comprising fixing a toner image to a transfer medium under application of heat and pressure by transporting a transfer medium having a toner image formed of a toner, so as to be held between a heating member and a pressure member; the former having a rotatingly movable heat-resistant film, wherein;
the pressure member comprises a mandrel and provided thereon with a heat-resistant elastic layer having a thermal conductivity of 0.20 W/mxc2x7K or below; and
the toner comprises toner particles containing at least a binder resin, a colorant and a wax, and an external additive; the toner having, in its DSC endothermic curve, at least a maximum value in the region of temperature of from 105xc2x0 C. to 150xc2x0 C., and having, in GPC of THF-soluble component of the toner, a main peak in the region of molecular weight of from 2,000 to 25,000, a sub-peak or shoulder in the region of molecular weight of 70,000 or higher, and an Mw/Mn ratio of 8 or higher, containing 20% to 70% of a component belonging to the region of molecular weight of from 1,000 to 10,000.